1. Field of the Invention
The present invention relates to a color deviation detection method, a color deviation detection device, a color deviation detection and correction method, a color deviation detection and correction device, a color image forming device and a process cartridge which make it possible to increase the reliability of color deviation detection, greatly reduce the error caused by the arrangement of marks in test patterns, and increase the precision of color deviation correction.
2. Description of the Related Art
For example, color deviation detection methods for detecting the positional deviation of pixels of a plurality of colors in color image forming devices are disclosed in Japanese Patent No. 2,573,855, Japanese Patent Application Laid-Open No. 11-65208, Japanese Patent Application Laid-Open No. 11-102098, Japanese Patent Application Laid-Open No. 11-249380, Japanese Patent Application Laid-Open No. 2000-112205 and the like. In these conventional color deviation detection methods, toner marks of respective colors are respectively formed in specified alignment patterns on a transfer belt (near both ends of this belt in the lateral direction) in which transfer paper is supported and conveyed along an arrangement of a plurality of photosensitive drums, and toner images of respective colors on the abovementioned plurality of photosensitive drums are transferred onto this transfer paper, the toner marks at the respective ends of the transfer belt are respectively read by a pair of optical sensors, and the positions of the respective marks of the mark arrangements (patterns) are calculated on the basis of these reading signals. Furthermore, the amount of deviation of the respective color images from a reference position in the sub-scanning direction (direction of movement of the transfer belt), the amount of deviation in the main scanning direction (lateral direction of the transfer belt), the amount of deviation of the effective line length of the main scanning lines and the skewing of the main scanning lines are calculated.
In the optical sensors, the reflected light or transmitted light of the transfer belt is received via slits by photo-electric conversion elements such as photo-transistors or the like, this light is converted into a voltage (analog detection signal) that indicates the amount of received light, and this voltage is corrected to a specified level range by an amplifier circuit. Accordingly, a detection signal of (for example) 5 V (high level: H) is obtained in cases where no marks are present in front of the abovementioned slits, and a detection signal of (for example) 0 V (low level: L) is obtained in cases where marks are present so that the entire surfaces of the abovementioned slits are covered.
However, since the transfer belt moves at a constant speed, the levels of the detection signals of the optical sensors gradually drop when the leading edges of marks enter the visual fields inside the slits of the optical sensors, the detection signals of the optical sensors remain at 0 V while the marks cover the entire surfaces of the slits, the levels of the detection signals of the optical sensors gradually rise when the trailing edges of the marks enter the visual fields inside the slits, and the detection signals of the optical signals return to 5 V when the marks have completely passed by the slits. This is an ideal case; in actuality, the detection signals of the optical sensors show a level fluctuation.
In cases where a level fluctuation is generated in the detection signals of the optical sensors, a binary signal distribution (with L corresponding to the marks) of a time series corresponding to the mark distribution is obtained by binarizing the detection signals of the optical sensors with (for example) the intermediate value of 2.5 V between 5 V and 0 V taken as the threshold value. Accordingly, the mark patterns can be grasped by binarizing the detection signals of the optical sensors by means of a comparator, counting clock pulses or timing pulses of a frequency that is proportional to the movement speed of the transfer belt, and storing the count value at the time that the output signal of the comparator changes from H to L and the count value at the time that this output signal changes from L to H in memory.
However, in the detection signals of the optical sensors, the level shifts during mark pattern detection and the changes in height with a relatively short period are large and numerous, and the level of the detection signals of the optical sensors also varies according to the color of the marks (type of toner). High-frequency noise of the detection signals of the optical sensors can be suppressed by passing the detection signals of the optical sensors through a low-pass filter; however, if the cut-off frequency is shifted toward a lower region in order to strengthen this suppression, the L pulse width corresponding to the mark width of the binary signals from the comparator shows an increased fluctuation in width, so that mark pattern recognition, and especially specification of the positions of the respective marks, becomes difficult. These problems become more severe with increasing contamination and scratching of the transfer belt, so that even if the useful life of the transfer application of the transfer belt is long, mark pattern detection for the purpose of color adjustment soon becomes impossible.
Accordingly, attempts have been made to identify data group positions corresponding to a reference waveform, and thus to recognize mark patterns, by repeatedly subjecting the detection signals of the optical sensors to an A/D conversion in a short period, collecting these converted signals in the memory, and performing a check of matching with the reference waveform or frequency distribution of the detection signals based on the data in the memory. In this case, however, the amount of data that is handled is greatly increased so that a large memory capacity is required; in addition, the pattern identification processing is complicated, and requires a long processing time.
Incidentally, the positions of the respective marks of the mark patterns in the movement direction of the transfer belt tends to fluctuate. For example, in cases where eccentricity or rotational irregularities are generated in the photosensitive drums or driving roller of the transfer belt, the positions of the marks show a deviation. A procedure in which marks of the same color are formed in two places at a pitch of one half of the circumference of the photosensitive drums, the amount of deviation of these positions with respect to a reference position is detected, and the mean value of this detected value is calculated as the amount of deviation, and in which such detection of the amount of deviation is further repeated a plurality of times (n times), and the mean value of 1/n is determined, in order to reduce the error of color deviation detection caused by such fluctuation in the positions of the marks, is disclosed in Patent Reference 2.
Furthermore, a procedure in which mark sets comprising arrangements of marks of respective colors are formed at a pitch of one fourth of the circumference, so that four sets are formed in the circumferential length of the photosensitive drums 1, the positional deviation of the respective marks on the transfer belt with respect to a reference position is detected following the transfer of these mark sets onto the transfer belt, and the mean value of the amount of positional deviation of the marks of the same color (four marks) is calculated, is disclosed in the abovementioned Japanese Patent Application Laid-Open No. 2000-112205.
Furthermore, if there is eccentricity in the photosensitive drums, the photosensitive drums show a maximum radius in a certain position, and show a minimum radius in a position located one-half circumference from this [maximum position]. In cases where there is elliptical distortion in the photosensitive drums, the position located one-half circumference from the position where a maximum radius is shown by the photosensitive drums is also a position where the radius is close to maximum. Accordingly, in a configuration in which marks of the same color are formed at a pitch of one half or one fourth of the circumference of the photosensitive drums, the averaging effect of the mean value is low. Specifically, the reliability of the measurements of the amount of deviation is low.
Furthermore, in the case of fluctuation components in products in which the circumferential length is longer than the total length of the plurality of mark sets, i.e., the pattern group length, measurements of the amount of deviation cannot be accurately performed using conventional pattern dispositions, so that correction that is conversely erroneous is commonly performed.
In the prior art, although the dispositions of the respective marks are taken into account for the respectively independent fluctuation waveforms with regard to fluctuations in the photosensitive body period and transfer belt driving roller period when calculating the mean values of the amounts of positional deviation of the images of respective colors, conventional methods do not go so far as to calculate the mean values of the amounts of positional deviation of the respective colors with respective marks disposed in the synthesized wave of these waveforms or a synthesized wave that includes the frequencies involved in the photosensitive body driving system and transfer belt driving system; accordingly, the precision of color deviation detection in such methods is low. Furthermore, the work of replacing the photosensitive bodies or developing devices is itself a cause of fluctuation in the color deviation, and color deviation caused by slight differences in the part precision before and after such replacement also occurs.
Furthermore, in the prior art, accurate measurements of the amount of deviation cannot be performed in the case of fluctuation components of products with a circumferential length that is longer than the pattern group length, so that there is on the contrary a possibility that an erroneous correction amount will be calculated. Conventionally, in order to avoid this problem at least to some extent, this has been countered by greatly improving the precision of one circumferential deviation of products with a long circumferential length. Naturally, this has contributed greatly to the cost involved, so that such parts are among the most expensive parts used in image forming devices.